A typical electrical device has terminals or pins. Cables or conducting traces on a circuit board are usually used as transmission lines connecting between the terminals of two or more devices to transfer signals between the devices.
To maintain the property of the signals during the transmission between the terminals and the transmission lines, each terminal of a typical electrical device usually has a termination resistor. The termination resistor often has a value chosen to match the resistance (impedance) value of the transmission line connected to the terminal to reduce signal reflection on the transmission line.
Changes in the environment factors such as manufacturing processes, voltage, and temperature may change the value of the termination resistor. Thus, various methods were developed to hold the value of the termination resistor at the chosen value when the environment factors changes.
In one known method, an external resistor is used as a reference to hold the termination resistor at the chosen value. However, the external resistor requires additional pins for the device. The additional pins waste available space and may increase the cost of the device. Further, even if the value of the termination resistor is kept at the chosen value using known methods, the resistance value of the transmission line may change due to changes in factors such as signal frequency and design topology of the transmission lines. Therefore, even if the value of the termination resistor remains at the chosen value, a mismatch may still exist between the value of the termination resistor and the resistance value of the transmission line.
The mismatch between the value of the termination resistor and the resistance value of the transmission line may cause degradation of the signal received at the terminal or may cause detection of the signal at the terminal more difficult or inaccurate.